* syssignal.h: Declare main_thread.

* syssignal.h: Declare main_thread.
(SIGNAL_THREAD_CHECK): New macro.

* keyboard.c (input_available_signal): Move thread checking code
to macro SIGNAL_THREAD_CHECK and call that macro.
(interrupt_signal): Call SIGNAL_THREAD_CHECK.

* alloc.c (uninterrupt_malloc): Move main_thread to emacs.c.

* emacs.c: Define main_thread.
(main): Initialize main_thread.
(handle_USR1_signal, handle_USR2_signal, fatal_error_signal)
(memory_warning_signal): Call SIGNAL_THREAD_CHECK.

* floatfns.c (float_error): Call SIGNAL_THREAD_CHECK.

* dispnew.c (window_change_signal): Call SIGNAL_THREAD_CHECK.

* sysdep.c (select_alarm): Call SIGNAL_THREAD_CHECK.

* process.c (send_process_trap, sigchld_handler): Call
SIGNAL_THREAD_CHECK.

* data.c (arith_error): Call SIGNAL_THREAD_CHECK.

* atimer.c (alarm_signal_handler): Call SIGNAL_THREAD_CHECK.